The present invention relates to an organic photoconductor used in the field of a copying machine and a printer, a process cartridge and an image forming apparatus both employing the organic photoconductor.
Compared with an inorganic photoconductor such as a selenium-based photoconductor and an amorphous silicone photoconductor, an organic photoconductor has great advantages including a broad range of selection for materials, an excellent aptitude for environment and low manufacturing cost, and the organic photoconductor has recently become a leading electrophotographic photoconductor, replacing an inorganic photoconductor.
In an image forming method based on Carlson method, on the other hand, an organic photoconductor is charged electrically, then, an electrostatic latent image is formed thereon, then, a toner image is formed therefrom, and the toner image is transferred onto a transfer sheet to be fixed thereon to become a final image.
A corona discharger representing a discharging member that has been used typically as a member of the aforementioned charging means is known exceedingly. The corona discharger has an advantage that stable charging can be carried out. However, the corona discharger needs to be applied with high voltage, and therefore, an amount of generation of ionized oxygen, ozone, moisture and nitrogen oxide is large, resulting in problems that the organic photoconductor (hereinafter referred to also as photoconductor) is deteriorated, and the human body is adversely affected.
Accordingly, there has recently been studied a use of a contact charging method that employs no corona discharger. Specifically, voltage is impressed on a magnetic brush and a conductive roller which are charging members, and these members are brought into contact with a photoconductor representing an object to be charged, so that a surface of the photoconductor is charged to the prescribed voltage. If the contact charging method of this kind is used, lower voltage can be used and an amount of ozone generation is less, compared with a non-contact charging method employing a corona discharger.
The contact charging method is a method to impress DC voltage on which DC or AC is superposed on a charging member having resistance of about 102-1010Ω·cm, then, to bring the charging member into pressure contact with the photoconductor to give it electric charges. Since this charging method is carried out by discharge from the charging member to the object to be charged, under Paschen's law, charging is started by impressing voltage that is not less than the threshold value. Compared with a corona charging method, voltage to be impressed on the charging member is lower, and an amount of generation of ozone and nitrogen oxide is less, in this contact charging method.
On the other hand, in the recent image forming method, digitizing has been advanced, and an image forming method using a laser beam as an exposure light source is commonly used for forming an electrostatic latent image of an organic photoconductor.
However, in contact charging method with a charging roller, if a support of organic photoconductor processed to prevent interference fringes (hereinafter referred to as moiré) caused by laser beam exposure is used, namely, if an aluminum support whose surface is roughened by cutting is used, there is caused a problem that dielectric breakdown is easily caused by convex portions on the surface subjected to cutting. Further, if the surface of the organic photoconductor is charged repeatedly, cracks and contamination are generated on the organic photoconductor, resulting in various troubles that electric charges are concentrated on a portion of cracks and contamination, image defects such as dielectric breakdown and black spots are easily generated, and blurred images tend to be caused. In particular, these problems tend to occur under the conditions of high temperature and high humidity, and low temperature and low humidity.
In addition, the organic photoconductor has a problem that its surface tends to be worn away because of friction against the contact charging member or a cleaning member. For preventing deterioration of this surface layer caused by abrasion, there is proposed a photoconductor wherein polycarbonate resin having high abrasion resistance, namely, polycarbonate resin in which a central carbon atom is a cyclohexylene group (which is known as polycarbonate Z (that is also called BPZ simply), is used in a binder for a charge transport layer (Patent Document 1).
However, if abrasion resistance of the organic photoconductor is improved by using the binder stated above, inorganic external additives such as silica in a developer is embedded in the surface of the organic photoconductor by pressure of the contact charging member, thus, the surface of the organic photoconductor tends to be contaminated by components of the external additives, and dash mark (small streak images in a form of a comet) and dielectric breakdown are easily caused, although abrasion resistance is improved.
As an organic photoconductor that is fit for the contact charging method, there is proposed an organic photoconductor having in its surface layer the fine particles of fluorine-containing resin for preventing image roughness caused by dielectric breakdown or scratches which result from the contact charging means (Patent Document 2). Further, as a photoconductor suitable for a contact type charging method, it has been proposed to provide a charge injecting layer in a photoconductor in which the charge injecting layer contains fluorine-containing resin particles, and it is reported that problems of unsharp image and scratching can be improved (Patent Document 3). However, in the organic photoconductor having in its surface the particles of fluorine-containing resin which has been reported so far, a fluorine-containing resin particle is easily separated from the binder on a boundary between them in the surface layer, thus, electric charges of the contact charging means tend to flow in the boundary surface, and dielectric breakdown and black spots are easily generated. Further, external additives of toner concerning the contact charging means easily adhere to the boundary surface between the fluorine-containing resin particle and the binder, and blurred images and dash mark (small streak images in a form of a comet) are generated, which easily deteriorates sharpness of text images and halftone images.
As a means to improve adhesion of foreign substances to a surface of the photoconductor, there has been reported a technology to use fluorine-containing resin particles having low crystallinity (a half band width of a peak of X-ray diffraction pattern is not less than 0.28) (Patent Document 4). However, in the fluorine-containing resin particles having low crystallinity, fluorine-containing resin particles tend to adhere to each other, dispersion stability in coating dispersion is low, the particles tend to cohere each other to form a coarse aggregated particle, resulting in problems that a surface layer having uniform characteristics is hardly formed, dash marks and blurred images are generated and halftone images are roughened.
(Patent Document 1) TOKKAISHO No. 60-172044
(Patent Document 2) TOKKAI No. 2002-278096
(Patent Document 3) TOKKAISHO No. 2003-5411
(Patent Document 4) TOKKAIHEI No. 8-328287